The application and characteristics of potassium fluoroaluminate flux for brazing, Shenyang Beizhen takes you to know, China’s professional vacuum furnace manufacturer, complete after-sales service, to protect your rights, Tel: +86 13309837263

Application of potassium fluoroaluminate flux

  • Potassium fluoroaluminate flux is not suitable for flame brazing. This is because the exhaust gas after combustion of combustible gas is mainly water vapor (and CO 2 ). The temperature of the flame is easy to hydrolyze KA1F4, so that the hydrolyzed product dehydrates A120, residue increase. At this time, a lot of flux is required to complete the brazing process, and the joint is not beautiful. Experienced welders often preheat the workpiece before applying the flux to the flux melting temperature and then apply the flux to minimize the contact time between the flame and the flux. Only in this way can a satisfactory process result be obtained_
  • Brazing in the furnace is the best way to get it. Brazing of the flux in dry air yields sufficiently satisfactory results. I smell the northern part of North China. From December to February of the next year, the relative humidity of the air is more than 25%. Direct brazing is completely feasible. Other seasons and areas should be brazed in a nitrogen-protected furnace. The key is to control the moisture in the atmosphere, followed by the oxygen content.

Modification of potassium fluoroaluminate flux

  • In the application of potassium fluoroaluminate fluxes, seven major publications have been published in recent years to discuss improvements in the Nocolok method. There are two aspects to the summary: one is to add a third or more salts to the potassium sulphate flux to increase the activity and other properties of the flux, and the other is to develop a new application method of *aluminum aluminate flux. .
  • The presence of silicon can make the box; the activity of the alumina acid evaluation flux is greatly increased. This is an unexpected effect of the existing impurities in the raw material. If you want to further improve the activity of the flux, the ideal method is in the form of KjSiF. Add people | 4n. The relationship between the K2SiF6 and AlFj-KF ternary systems can be found in AlF3-KF-SiF published by AjwpHftKO, phase diagram 1<2j.
  • When Si) exceeds 2% of the total amount of the flux, the alloy formed by the reaction of the precipitated Si and the base material can be self-brazing without additional solder. Adding K2GeF6 will have higher activity, and adding ZnF2, PbF2, ZnF2w or KBF4 can effectively improve the activity of the flux. This is obvious because the cations of these additives are reduced and precipitated by the base material during brazing, and alloyed with the base material to form a liquid metal layer, which acts as a mass transfer. However, if the flux contains the most ruthenium metal ions, the color of the joint will be darkened during brazing, and the heavy metal precipitated in C will be too much for a while, and it will not be alloyed with the base material to form fine particles suspended in the flux. In the presence of boron, experiments have shown that the joints tend to be heavily blackened because the reduced boron cannot be alloyed with the parent metal at all, as can be seen from the phase diagram of A1-B.
  • In the improvement of the Noculok method, some patent documents reported that the potassium fluoroaluminate flux was mixed with some alloy powders, for example, by using zinc powder 147, Al-Zn alloy powder W’501 and flux to prepare a slurry for spraying. The surface of the T piece is heated and melted to form a corresponding Zn layer or Al-Zn layer on the surface of the workpiece to facilitate a small amount of brazing.
  • In recent years, KA1F has been gradually developed as a gas phase agent: one is to mix the vapor of KA1F4 in a low-pressure non-oxygen atmosphere to braze the aluminum alloy [M], and the other is The outside of the aluminum alloy parts is vacuum deposited – layer KA1F4 [52], and then assembled and brazed as needed.
  • In the I’loating method of the ANSi eutectic nail powder, a layer of KA1F is deposited to form a ruthenium-pulverizer complex: 0:, and then an organic solvent such as n-nonanol is used. Adjust to use for welding.
  • One disadvantage of the potassium sulphate flux is that the rubbing temperature is unfavorable for the operation of the Vacuum brazing furnace process. A monumental scholar explored ways to reduce the temperature of MttVKAlF, the eutectic point. Since all ionic fluorides are incapable of dissolving the liquid K3A1Fo-KAU”’* molten salt’, some of the covalently structured vapors are soluble. Figure 2-13^ shows the molten salt phase diagram of aif3-kf-kci
  • This system has three ternary eutectic points: where the A and A eutectic temperatures are 534t and Mlt, respectively. This is very welcome compared to the lower melting temperature of Nocolok flux / 24 and 丨7.
  • Unfortunately, the viscosity of the molten salt is greatly increased after the addition of KC1, and the high fluidity and spreadability of the NocoWc flux are lost. However, this molten salt is soluble in water, so that the molten slag is washed, and it is not absorbed at all in moist air. If it is used as a matrix, a certain amount of surfactant is added, and SI can be used for its practical application.
  • Liu Zan et al. experimented on the determination of the melting temperature after adding different copper KBr to the A1F3-KF eutectic. Experiments have shown that when the content of KBr is 20% (mass S fraction), a molten salt having a melting temperature of 522 Å can be obtained. Wenyin believes that the formation of KA1F4-K in the flux, AlK6-KAlBr4 ternary eutectic is the main reason for the melting point of the flux, and the brazing effect is good.

Medium temperature fluoroaluminum strontium salt flux

  • The biggest disadvantage of the conventional Nocolok method for the application of potassium fluoroaluminate flux is that the operating temperature (600 VII) is too high, which is also limited by the flux of the flux and the nail melting temperature (5581 and 577*C, respectively). The alloy cannot be brazed with a potassium fluoroaluminate flux because the overheating temperature is lower than 6001. The so-called over-burning temperature generally refers to the temperature of the solidus line exceeding the alloy (see Table 2-3). When the alloy is heated to this temperature, the molten phase begins to appear in the intergranular phase, causing damage to the structural structure. Figure 2-14 shows the over-burning temperature of the main aluminum alloy [~. It can be seen that in addition to hard aluminum (2x x x series) and (7 x x x series), if the brazing temperature is less than 540 ~ 550X, a considerable amount of alloy is safe. If brazing hard aluminum is to be considered, the brazing temperature cannot exceed 500 弋. In this way, the melting temperature of the flux is required to be lower than 480 – 490T. Therefore, it is required to develop a flux which is non-corrosive, hardly soluble in water and has a melting temperature of less than 480 to 490, which is an important factor in the development of aluminum flux for many years. aspect.
  • Based on the potassium aluminosilicate Nocolok flux, it is a matter of course to add a third fluoride component to lower the melting temperature of the potassium aluminosilicate. In fact, in the molten salt potential sequence (see Table 2-5) A1, the elemental telluride, including rare earth, titanium, zirconium and other gold fluoride can not be used as the main component of the flux. Experiments have shown that when there are few vapors of these most elements, they can sometimes act as active agents. When they are used as the main component to lower the melting temperature of Nocolok, the flux is too large. It will be reduced by aluminum to precipitate a large amount of metal particles, and the surface of the base material will be made worse. In fact, only the alkali fluoride and alkaline earth metal fluorides are available for B.
  • According to a recent study, KA1F4 in the melt is mainly in a molecular state when 1^?4 is melted. The fluoride of the ion, including MgF2, CaF2, SrF2, BaF2, and even LiF containing a part of the covalent bond, is hardly soluble therein and has no effect on the melting temperature of the potassium fluoroaluminate flux. Only the molecular type of BeF2 and other molecularly-formed chromium fluorides can significantly reduce the potassium fluoroaluminate eutectic temperature. The AlF,-KF-BeF2 phase diagram has a more detailed report [56,n], some of which are attractive. For example, the melting temperature of the eutectic point of the KA1F4-KBe2F5赝 binary system is only 335. However, due to the highly toxic compounds, the results of this study are difficult to have practical significance except in special cases [5<1.
  • Th0maw published a report on the phase diagram of the molten salt of KF-AlF, -I, and iF. The phase diagram indicates that the composition is ic(KF) = 43_0%, «o(A1F3) = 54.7%, and w(LiF) = 2. 3% is a three-twisted eutectic point with a temperature of 490 。. According to Thoma’s phase diagram, Yamawaki M’s declared the patent protection for this component flux. However, our experiments have proved that Yamawaki’s patents are not credible at all, and even the maps of Thomas are doubtful.
  • Despite this, according to experienced work technicians, although LiF has not been able to reduce the melting temperature of the flux, it is possible to add 1% – 丨.5% (mass fraction) of UF in Nocolok flux. It can reduce the amount of flux, prolong the activity time, and is easier to process. Therefore, the addition of LiF often becomes the latent process of these Nocolok flux units. For this reason, there are also some manufacturers of Nocoiok flux, which have not been marked, but have added about 1% (quality of children) I, iF to the product.
  • A medium-temperature insoluble flux with reliable application prospects is Su-zuki (61: application to CsF-A1F3 system. The melting point of this flux is 440 – 480^, and the patent protection component is *(A1F3) = 26 % -67% , *(CsF) =74% ~ 33%. It is pointed out that this flux has a relatively high brazing efficiency and is more stable to flame than fluoroaluminate. It is said that the biggest advantage is that it contains The magnesium alloy has a special activity. The phase diagram of CsF-A1F3 is shown in Figure 2-15 [621. The composition of the eutectic point used as flux in the figure is *(A1F3) = 42.0%, *( CsF) = 56.0%, melting temperature is 4711:0.
  • Comparing the phase diagrams of CsF-A1F3 and KF-A1F3, it can be seen that KA1F4 is a solid-liquid aliquot compound with good thermal stability; while CsA1F4 is a solid-liquid equivalent The compound has poor thermal stability and decomposes when heated to 4431. The point A in the CsF-A1F3 system using the formulated flux is actually a eutectic of Cs3A1F6 and CsA^F, (ie C3F-2A1F,). KA1F4 can be used as a flux alone, but C»A1F4 is not. CsjAIF6-CsA1jF7 eutectic molten salt is also much more soluble in water than Nocolok, which is mainly affected by CsA12F7, its dissolution About 0.8% ~ 1.0% 0 However, it is not hygroscopic in the air, and the residue of brazing is not corroded. “Instead, the residue of this flux is much easier to wash than the residue of Nocolok. The residue can be cleaned with cold water.
  • In order to further reduce the melting fluency of the above A eutectic point flux, and in order to reduce the content of expensive CsF, the patent of A1F3-KF-CSF ternary flux was continuously published from 1986 to 1987, because there is no relevant The guidance of the phase diagram, the scope of protection of this patent application is very wide. The patent states that a flux with a minimum melting temperature of 460 is obtained, which is said to have a high activity for the most Mg-containing aluminum alloy. T is suitable for brazing all aluminum alloys.
  • The A1K3-KF-CsK-Metaline Ocean Fine Phase Diagram was published shortly before fm<7] (see Figure 2-16). The structure of this phase diagram is more complicated. The composition and temperature of each zero-change point in the figure are shown in Table 2″*. The zero-change point refers to the phase change point with zero degree of freedom, where the melting interval of the molten salt is zero. And the table can be seen that & 455 <0, ~ (46 («:) are very desirable composition points. The lowest temperature component point is &, the phase diagram is not finely measured, the estimated melting M degree should be 430 E, The melting temperature of £5 is 478T, which is roughly the same as the melting temperature of 471<c of the CsF-A1F3 binary system, but it is much less than the CsF used, and only 〜/3 is used. It is more economical to formulate flux at two component points.
  • The point in Figure 2-16 is the eutectic point in the K3A1F6-KA1F4-(2CSjA1F6. 3KA1FJ ternary system with a melting temperature of 478t). These three compounds are all solid-liquid equivalent (see Reference [66]). The thermal stability is very good. e9 is the eutectic point of KA1F4-(2Cs3A1F6, 3KA1F4)  binary system, the melting temperature is 480T, and has similar characteristics. Secondly, it is optional £3 (5101), which is more stable. K3A1F6-Cs, A1F6-(2C83A1F6 • 3KA1F4) 共 eutectic point in the ternary system. Therefore, if a manufacturer attempts to plan a series of such aluminum flux products, then the composition of their flux should of course be 🙁 558^ ) ; E3 (510弋);( 478T. ) ; Et (455^); £,(«430’C) o

Table 2-8 Nonvariant points in the ternary system AlF3-KF-CsF

Composition (molar fraction, %) Temperature / especially
3.0 59.0 38.0 550
8.0 77.0 15.0 608
35.0 39.0 26.0 510
43.0 18.0 39.0 478
e5 45.0 18.0 37.0 478
40.0 50.0 10.0 455
42.0 34.0 24.0 525
*1 57.0 43.0 625
7.0 93.0 820
44.5 59.5 558
10.0 90.0 654
42.0 58.0 471
8.3 66.8 24.9 610
37.0 25. 1 37.9 550
37.6 36.2 26.2 550
43.4 18.5 1 38. 1 480
42. 1 28.5 29.4 550
44.0 38. 1 17.9 530
40.0 51. 1 8.9 460
50.0 15.0 35.0 510
50.0 29.0 21.0 525
25.0 56.0 19.0 775
% 50.0 45.0 5.0 450
  • The price of Csl•’ is very high, and this type of flux is missing. The relative molecular mass of CsF is very large, about 152. When converted to a qualitative tt fraction, the proportion of CsF in the flux seems to be larger. So people’s attention turned to the AlF3-RbF system. The phase diagram of this system is shown in Figure 2-7 [M]. The composition of £2 in the system is *( RbF) = 51.6%, *(A1F3) = 48.4%, and the melting temperature is 486T, which is an optional flux composition. The relative molecular mass of RbF is 104.5, which is much more than the CsF flux, and the price of the two is comparable. Synthetic CsF or RbF fluxes can use their carbonates as raw materials, which are slightly more stable than strontium hydroxide. Although they do not absorb ≥2 like strontium hydroxide, they also absorb water heavily. Therefore, it should be 120 before weighing. ~ 130 弋 thoroughly dehydrated. The synthesis can be carried out according to the method of Nocolok &C03, and there is nothing special. Si impurities are a “naturally-friendly” active agent in AIF3-KF flux, but Si impurities cause great trouble in CSF or RbF flux. Nocolok AIF3-KF flux has a higher melting temperature (558T). When the flux is melted, the element Si reduced by the aluminum base material is immediately alloyed with the base metal to form a shiny melt on the surface of the base material. The alloy layer, this process has an effective activation. The melting temperature of the CSF or RbF flux is relatively low, and the reduced Si after the flux is melted cannot be alloyed immediately with the base material, but suspended in the flux in the form of fine particles, which makes the center of the flux spread black. . Domestic commodity chemically pure CsjCO; or Rb2C03 is well labeled on the surface of the cup containing the K, but the content of silicon is often not marked. In fact, the content of silicon often exceeds 0.1% (mass fraction). Dissolve Cs2C03 or Rb2C03 in pure water. If the turbidity is unclear, the Si content is too high. This is because the impurities in the form of Cs2SiO are hydrolyzed to silica gel and suspended. This silica gel cannot be removed by filtration. Therefore, when using this raw material, it is necessary to repeatedly add excess HF in a polytetrafluoroethylene vessel and heat to remove it. Si. Therefore, when selecting raw materials, pay great attention to the silicon in the carbonate and the impurity content of iron.